April 3

National Find a Rainbow Day

An annual observance on April 3 encouraging people to look for rainbows in nature and appreciate the optical phenomenon created by sunlight refracting through water droplets.

Yearly Date

April 3

Observed in

United States

Introduction

Most people spot a rainbow by accident. National Find a Rainbow Day suggests doing it on purpose, and once you understand the geometry, it becomes surprisingly predictable. The sun must be lower than 42 degrees above the horizon, the rain must be in front of you, and you must be positioned between the two. That precision is why most rainbows appear in early morning or late afternoon.

The science behind them is equally specific. A rainbow is not an object with a fixed location. It exists only as a relationship between sunlight, water droplets, and your exact position. Move a few steps and the rainbow shifts, because you are now seeing light refracting through a different set of droplets. Every rainbow is personal.

National Find a Rainbow Day History

Rainbows have appeared in mythology longer than anyone has understood what causes them. In Greek mythology, Iris was the goddess of the rainbow, serving as a messenger between the gods and humanity. In Norse mythology, the Bifrost was a burning rainbow bridge connecting Midgard (Earth) to Asgard (the realm of the gods). The Hebrew Bible describes the rainbow as a sign of God's covenant with Noah after the flood.

These stories all treated the rainbow as something placed in the sky with intention. Understanding what actually creates one took much longer.

From reflection to refraction

Aristotle proposed around 350 BCE that rainbows resulted from the reflection of sunlight off clouds. The explanation was wrong, but it stood largely unchallenged for over 1,500 years. The first real breakthrough came in 1304, when Theodoric of Freiberg, a Dominican friar, filled glass spheres with water and traced the path of light through them. He showed that light refracts when entering a water droplet, reflects off the inner surface, and refracts again on the way out.

Rene Descartes built on Theodoric's work in 1637, publishing a mathematical proof that light exits a raindrop at approximately 42 degrees relative to the incoming ray. This explained the consistent angular size of rainbows but left one question unanswered: why the colors?

Newton's prism

In 1666, Isaac Newton answered that question at Trinity College, Cambridge. Using a glass prism, he demonstrated that white sunlight is not a single thing but a mixture of wavelengths, each bending at a slightly different angle when passing through a transparent medium. Red light bends the least; violet bends the most. A raindrop works exactly like Newton's prism, separating sunlight into the spectrum we see arced across the sky.

Newton's work completed the scientific explanation that Theodoric and Descartes had begun. National Find a Rainbow Day appeared on online holiday calendars around 2015, observed on April 3. No founder has been documented. The date falls in early spring, when rain showers followed by clearing skies create frequent conditions for rainbow formation across much of the United States.

National Find a Rainbow Day Timeline

~350 BCE

Aristotle describes rainbow theory

Aristotle proposed that rainbows were caused by the reflection of sunlight from clouds. His theory dominated Western understanding of optics for nearly two thousand years, even though it did not account for the separation of light into distinct colors.

1304

Theodoric of Freiberg explains refraction in raindrops

The German Dominican friar Theodoric of Freiberg conducted experiments with glass spheres filled with water and demonstrated that rainbows result from light refracting as it enters a raindrop, reflecting off the inner surface, and refracting again as it exits. His work anticipated modern rainbow theory by three centuries.

1637

Descartes calculates the geometry of rainbows

Rene Descartes published a mathematical explanation in his 'Discours de la methode,' showing that light exits raindrops at approximately 42 degrees relative to incoming sunlight. This explained why rainbows always appear at the same angular size but did not account for the spectrum of colors.

1666

Newton separates white light with a prism

Isaac Newton conducted his prism experiments at Trinity College, Cambridge, demonstrating that white light is composed of a continuous spectrum of colors. This completed the scientific explanation of rainbows: water droplets separate white sunlight into its component wavelengths, with each color bending at a slightly different angle.

~2015

National Find a Rainbow Day appears online

National Find a Rainbow Day began appearing on online holiday calendars, observed on April 3. No founding person or organization has been documented. The date falls in early spring, when rain showers and clearing skies create frequent conditions for rainbow formation in many parts of the United States.

How to Celebrate National Find a Rainbow Day

1

Learn a rainbow's geometry

The SciJinks rainbow guide from NOAA and NASA explains how sunlight bends inside raindrops and why each color exits at a different angle. Understanding the 42-degree rule makes spotting rainbows predictable rather than lucky.
2

Make a rainbow with a garden hose

Stand with your back to the sun and spray a fine mist from a garden hose. A rainbow will appear in the mist at the same 42-degree angle it appears in rain. This is the simplest way to demonstrate that the observer's position, not the water's location, determines where a rainbow forms.
3

Look for a double rainbow

Double rainbows form when light reflects twice inside a raindrop. The secondary arc appears at 50-53 degrees with its colors reversed: red on the inside, violet on the outside. The sky between the two arcs, called Alexander's band, appears noticeably darker than the sky above or below.
4

Read about the optics of color

The Britannica entry on rainbows covers the physics from Descartes to Newton and explains advanced phenomena like supernumerary bows, fogbows, and moonbows.
5

Photograph a rainbow correctly

Use a polarizing filter to increase color saturation, underexpose by one stop to prevent washing out the colors, and include foreground elements for scale. The best rainbow photographs are taken within 30 minutes of sunrise or sunset, when the arc reaches its maximum height.

Why We Love National Find a Rainbow Day

A

Rainbows require precise geometry to appear

A rainbow is visible only when the sun is lower than 42 degrees above the horizon and the observer is positioned between the sun and rain-filled air. This is why rainbows appear most often in early morning or late afternoon, and why they are full semicircles near sunrise and sunset but smaller arcs at midday. From an airplane, with the sun directly behind and rain below, a rainbow forms a complete circle.
B

Hawaii sees more rainbows than anywhere on Earth

The combination of frequent brief showers, clean mountain air with minimal pollution-related aerosols, and consistent sun angles makes Hawaii the rainbow capital of the world. Researchers at the University of Hawaii have documented this, and the state's Department of Transportation even registered 'The Rainbow State' as an official motto for its license plates.
C

Climate change may increase rainbow frequency

A 2021 study published in Global Environmental Change projected that land locations on Earth will experience at least 5% more rainbow-visible days by 2100. The increase is driven by shifting rainfall patterns: areas gaining more precipitation and more sun-rain overlap will see more rainbows, while regions experiencing prolonged drought will see fewer.